1. Field of the Invention
The present invention relates generally to an electric power steering apparatus for applying power from an electric motor to a steering system to lighten the steering force that must be exerted by a driver, and more particularly to an electric power steering apparatus which when an ignition switch is switched off and the supply of a steering assist force is stopped causes the steering assist force to decrease gradually with time.
2. Description of the Related Art
Reference is made to FIG. 4 hereof which shows in block diagram the construction of a control unit in a known electric power steering apparatus.
This control unit 120A has a target current calculating part 121, a drive control signal generating part 122A and a motor driving part 123. The target current calculating part 121 determines a target assist torque on the basis of a steering torque signal Tp outputted from a steering torque detector 112 and outputs a target current signal IT required for this target assist torque to be supplied from a motor 110 to a steering system. The drive control signal generating part 122A generates a PWM (Pulse Width Modulation) signal for PWM-operating the motor 110 on the basis of the target current signal IT, and outputs the generated PWM signal as a drive control signal 122a.
The motor driving part 123 has a gate driving circuit part 124 and a motor driving circuit 125 consisting of four power field effect transistors connected in an H-type bridge. The gate driving circuit part 124 drives the gates of the field effect transistors and thereby drives switching of the field effect transistors on the basis of the drive control signal (PWM signal) 122a. The control unit 120A PWM-controls power supplied from a battery power source BAT to the motor 110 on the basis of the steering torque signal Tp outputted by the steering torque detector 112 and thereby controls the output power (steering assist torque) of the motor 110.
FIG. 5 hereof is a block diagram showing the construction of another known control unit.
In the control unit 120B shown in FIG. 5, the motor current IM actually supplied to the motor 110 is detected and the control characteristic of the motor 110 is improved by feedback control based on the motor current IM being carried out. In addition to the construction shown in FIG. 4, this motor current feedback control type control unit 120B has a motor current detector 126 for detecting the current flowing through the motor 110 and outputting a motor current signal IM, a deviation calculating part 127 for obtaining the deviation between the target current signal IT and the motor current signal IM, and a drive control signal generating part 122B for outputting a drive control signal 122a for driving the motor 110 based on a deviation signal 127a outputted from the deviation calculating part 127.
For the motor current detector 126, a detector which using a Hall device or the like detects the motor current flowing through an electric wire to the motor without cutting that wire or a detector which detects the motor current by amplifying and converting into a current a voltage produced across a low resistance for current detection is used.
The drive control signal generating part 122B comprises a current feedback (F/B) control part 128 for carrying out proportional, integral or differential processing or the like on the deviation signal 127a and generating and outputting a drive current signal 128a for so controlling the current supplied to the motor 110 that the deviation approaches zero and a PWM signal generating part 129 for generating a PWM signal for PWM-operating the motor 110 on the basis of the drive current signal 128a and outputting the generated PWM signal as a motor drive control signal 122a.
Because this motor current feedback control type control unit 120B so controls the operation of the motor 110 that the deviation between the target current signal IT and the motor current IM actually supplied to the motor 110 approaches zero, it can have increased control accuracy and responsiveness.
However, in the electric power steering apparatus described above, when an ignition switch IGN is switched off while the steering wheel is being operated, because the steering assist torque being supplied from the motor suddenly falls to zero, the steering wheel may be moved by a reaction from the tires, and this is undesirable. Also, when the ignition switch is switched off and the steering assist torque suddenly falls to zero, the steering wheel suddenly becomes heavy, and this may be disconcerting for the driver.
As a countermeasure to these problems, in Japanese Patent Laid-Open Publication No. SHO 62-181958, the present inventors have proposed an electric power steering apparatus with which when the ignition switch is switched off or a failure arises in the control unit, the steering assist torque supplied from the motor is reduced gradually (fadeout-controlled) and the steering force that must be applied to the steering wheel by the driver is thereby increased gently so that a good steering feel is obtained.
This electric power steering apparatus comprises a steering state detector for detecting a steering state of the steering system, a motor control signal generator for generating and outputting a motor control signal on the basis of a steering state detection signal from the steering state detector, and motor driving means for driving a motor on the basis of the motor control signal from the motor control signal generator, and also has an operation stoppage detector for detecting a stoppage in the operation of the apparatus itself and a corrector for causing the motor control signal to diminish gradually with time on the basis of a detection signal from the operation stoppage detector.
FIG. 6 hereof is a block diagram of a known control unit constructed to gradually reduce the steering assist torque supplied from a motor when an ignition switch is turned off (that is, to carry out fadeout control).
This control unit 120C consists of the control unit 120A shown in FIG. 4 with a correcting part 131 interposed between the target current calculating part 121 and the drive control signal generating part 122A. During normal operation, in which fadeout control is not carried out, this correcting part 131 feeds the target current signal IT supplied to it from the target current calculating part 121 unchanged to the drive control signal generating part 122A.
When the ignition switch IGN is turned off or when a failure of the control unit has been detected by a failure detecting part (not shown) or when it has been detected by a battery voltage monitoring part (not shown) that the voltage of the battery BAT is above a permissible upper limit value or is below a permissible lower limit value, a fadeout request signal 131a is supplied to the correcting part 131 from an ignition switch monitoring part (not shown) or the failure detecting part or the battery voltage monitoring part. When a fadeout request signal 131a is supplied to it, the correcting part 131 generates a corrected target current signal ITH in which the target current value supplied from the target current calculating part 121 is gradually reduced and finally reaches zero and supplies this corrected target current signal ITH to the drive control signal generating part 122A.
As a result, when the ignition switch IGN is switched off or when the battery voltage is outside a predetermined voltage range, the steering assist force supplied from the motor 110 can be gradually lowered and the steering wheel being moved by tire reactions and the steering feeling suddenly changing can thereby be lessened.
FIG. 7 hereof is a block diagram of another known control unit constructed to gradually reduce the steering assist torque supplied from a motor when an ignition switch is turned off (i.e. to carry out fadeout control).
This control unit 120D consists of the control unit shown in FIG. 5 with a correcting part 131 interposed between the target current calculating part 121 and the deviation calculating part 127. During normal operation, in which fadeout control is not carried out, this correcting part 131 feeds the target current signal IT supplied to it from the target current calculating part 121 unchanged to the deviation calculating part 127.
When the ignition switch IGN is turned off or when a failure has been detected by a failure detecting part (not shown) or when it has been detected by a battery voltage monitoring part (not shown) that the voltage of the battery BAT is above a permissible upper limit value or is below a permissible lower limit value, a fadeout request signal 131a is supplied to the correcting part 131 from an ignition switch monitoring part (not shown) or the failure detecting part or the battery voltage monitoring part. When a fadeout request signal 131a is supplied to it, the correcting part 131 generates a corrected target current signal ITH in which the target current signal IT supplied from the target current calculating part 121 is gradually reduced and finally reaches zero and supplies this corrected target current signal ITH to the deviation calculating part 127.
When at this time a predetermined voltage is being supplied to the motor current detector 126 and the operation of the motor current detector 126 is normal and the current being supplied to the motor 110 is being correctly detected, the current supplied to the motor 110 is gradually decreased to zero along with the diminishing of the target current signal IT.
However, in the current feedback control type control unit 120D shown in FIG. 7, when the voltage of the battery BAT has fallen to below a voltage range in which the motor current detector 126 can operate normally, the motor current value IM being supplied from the motor current detector 126 may deviate from the current actually being supplied to the motor 110. In particular, even when the performance of the battery BAT has fallen, when the ignition switch IGN is on and the engine is running, because a generator is operating, power within a predetermined voltage range (for example 12 to 14 volts) is supplied from the generator to the various loads. However, when the ignition switch IGN is turned off and the engine stops, because there ceases to be any output of power from the generator and power is supplied to the loads from the battery BAT, if the performance of the battery BAT has fallen, the voltage supplied by the battery may fall (for example to below 8 volts) to far below its nominal voltage (for example 12 volts) and it may become impossible for the motor current detector 126 to correctly detect the value of the motor current.
Consequently, in the control unit carrying out current feedback control, when the ignition switch has been turned off or when it has been detected that the voltage of the battery BAT is above a permissible upper limit voltage or below a permissible lower limit voltage, even if the target current signal IT is corrected and the target current value IT is gradually reduced, if the voltage of the battery BAT has fallen to below the voltage range over which the motor current detector can operate correctly, the motor current IM cannot be detected correctly and because of this the current supplied to the motor 110 cannot be reduced in correspondence with the target current value. Consequently, when the ignition switch is switched off with the steering wheel turned, the steering wheel may move or the steering torque may suddenly increase, disconcerting the driver.
As a countermeasure to this, it is conceivable to improve the motor current detector 126 so that current detection accuracy can be ensured even when the power supply voltage supplied to the motor current detector 126 falls. However, when the current detection accuracy at low voltages (for example 7 to 9 volts) is increased, the current detection accuracy at normal voltages (for example 10 to 14 volts) and high voltages (for example over 14 volts) may fall. And when the range of operating power supply voltages of the motor current detector 126 is increased, the cost of the motor current detector 126 tends to increase.
It is therefore demanded that there be provided an electric power steering apparatus which detects the current supplied to a motor with a current detector and carries out feedback control of that current and makes the steering assist force decrease gradually when stopping the supply of a steering assist force when an ignition switch is switched off or the voltage of a battery power supply has fallen wherein even when an error has arisen in the current value detected by the current detector due to the fall in the voltage of the battery power supply the steering assist force can be suitably reduced and the motor current controlled to zero.